This invention relates to a high-vanadium (high-V) austenitic heat-resistant alloy with improved overall corrosion resistance and pitting corrosion resistance. More particularly, this invention relates to such an alloy suited for use in equipment which may be operated in severe environmental conditions such as those which may exist at coal gasification plants.
A high-temperature reducing atmosphere of 500.degree. to 700.degree. C. containing HCl and/or H.sub.2 S may be found, for example, in superheater tubes used in coal gasification plants. When such a plant is shut down, a wet corrosive environment may present itself. An alloy for equipment for use in such a plant is required to have both superior high-temperature corrosion resistance and superior overall surface corrosion resistance as well as aqueous corrosion resistance.
It has been known that the chromium (Cr) content of an alloy must be increased in order to effectively improve the corrosion resistance of materials made of such an alloy in high-temperature reducing atmospheres. It has also been known that addition of molybdenum (Mo), in addition to an increase in the Cr content, is an effective way to improve corrosion resistance in wet corrosive environments. Since Mo is detrimental to corrosion resistance in high-temperature reducing atmospheres, however, the addition of Mo is not practical for equipment such as superheater tubes in coal gasification plants.
It was recently disclosed by W. T. Bakker and R. A. Perkins ("Corrosion," NACE International Forum, Paper No. 525 (1989)) that vanadium (V) is not only effective in improving the corrosion resistance of alloys in wet corrosive environments but also capable of improving their corrosion resistance in high-temperature reducing atmospheres. The structural stability and high-temperature strength of high-V austenitic alloys after a long-term high-temperature service, however, have not been sufficiently investigated. One of primary reasons why these materials have not been utilized in high-temperature equipment may be their inferior corrosion resistance in oxidizing atmospheres. Another may be the fact that V is an element having a strong propensity to promote ferrite, and that it also promotes the precipitation of intermetallic compounds, typified by the sigma (.sigma.) phase.
Thus, although it is necessary to increase the Cr content in the alloy and a very large quantity of V must be added to such a high chromium austenitic alloy, it is a major problem to maintain structural stability when the alloy is used for long periods of time at high temperatures. Carbides and nitrides, which contain large quantities of V, tend to precipitate out during use in high-temperature environments if large quantities of V are added. This is because V has strong affinity to C and N, and this has the adverse effect of reducing the quantity of solid solution V, thereby also reducing the corrosion resistance. The precipitated vanadium carbonitrides may affect the toughness and creep rupture strength during a long-term exposure.